1. Technical Field
The present invention relates to the structure of an illumination system suitably used in an electro-optic device.
2. Related Art
In current electronic apparatuses, such as a mobile phone, a personal digital assistant, and a computer display, liquid crystal display devices are in widespread use as an electro-optic device for displaying a video image.
In such a liquid crystal display device, a backlight system (illumination system) is provided adjacent to the rear surface of a liquid crystal display panel so that the rear surface of the liquid crystal display panel is illuminated to perform transmissive display. Such illumination systems are classified broadly into two types, i.e., the direct-lighting type and the edge-lighting type depending on the position of a light source relative to the liquid crystal display panel.
In an illumination system of the direct-lighting type, a light source is located adjacent to the rear surface of a liquid crystal display panel. On the other hand, in an illumination system of the edge-lighting type, a light guide plate guiding light is placed adjacent to the rear surface of a liquid crystal display panel and a light source is located adjacent to a side surface of the light guide plate. The direct-lighting illumination systems do not need a light guide plate as distinct from the edge-lighting illumination systems. Accordingly, the direct-lighting illumination systems have an advantage in that the cost is reduced as much as a light guide plate. Direct-lighting backlight systems are disclosed in, for example, JP-A-2006-133708, JP-A-2006-301209, JP-A-2007-73295, and JP-A-2007-80798.
The direct-lighting backlight systems disclosed in JP-A-2006-133708, JP-A-2006-301209, JP-A-2007-73295, and JP-A-2007-80798 each include a substrate and a plurality of light emitting diode elements mounted on the substrate. In these backlight systems, the thickness of a light source unit including the substrate and the light emitting diode elements is equal to the sum of the thickness of the substrate and the height of the light emitting diode element.
Accordingly, in each of the above-described backlight systems, the thickness of the light source unit is large. It is therefore necessary to provide a space capable of receiving the light source unit. Disadvantageously, it is difficult to reduce the thickness of the backlight system.
A typical direct-lighting type backlight has a structure in which a light source unit is placed in a chassis made of resin or metal. When the luminance of light sources is increased, the amount of heat generated from each light source is also increased, thus increasing a temperature in the chassis. This causes a phenomenon in which the light emission efficiency of the light source is reduced.
In particular, in the use of LEDs as light sources, the number of light sources necessary for increasing the luminance is increased. Thus, the amount of heat generated is also increased. It is therefore necessary to devise a mechanism for cooling the LEDs or restrict the amount of current flowing into the LEDs.